Digital transmission of data requires a common time base for decoding the data. This determines when the data are valid in a receiver and can be sampled. If, in the case of message-oriented transmission, data packets are intended to be transmitted only upon request, the starting time for the transmission is required. This requires a further signal line if this information is not simultaneously encoded by means of the clock information. In embedded systems, this is achieved by means of a common system clock and selection lines which address data sources specifically.
In decentralized systems, multiwire buses are used, e.g. SPI (Serial Peripheral Interface), which is supported by many microcontrollers. In this case, the clock for the data
transmission is sent on a separate line for the purpose of bit-synchronous data transmission. A selection line indicates the start and the end of the data transmission.
If the number of lines needs to be reduced for reasons of 35 cost, the transmitter's clock and the starter of a transmission need to be encoded in the data to be transmitted themselves in order to allow reliable decoding in a receiver.
Economizing on external decoding hardware which needs to be used in addition to a microcontroller is an important cost factor which needs to be taken into account.